1. Field of the Invention
The invention relates to methods for encoding and transmitting data or information; more specifically the invention is a method for inverting distortion processes in a telecommunication channel.
2. Current Art and Problem Solution Needed
Recent information technology advancements such as real-time multimedia applications and high-speed Internet access, as well as the integration of common communications means such as telephone, Internet, television, and digital data systems have led to an unprecedented need for high-speed data transfer. However, many segments of today's wired networks are incapable of supporting the transfer rates required by these technologies. In many cases, it is impractical, both physically and economically, to install the necessary fiber optic and other broadband infrastructures to overcome this deficiency. Furthermore, in certain circumstances such as emergencies and military operations requiring high-speed data transfer, there exists an inherent need for rapid and temporary network access deployment, precluding fiber and traditional wireless systems, which have rather long lead-times.
An examination of existing legacy landline communications networks in light of communications technology evolution leads to some interesting observations. On the one hand, the newest long haul communications and information infrastructures being built today are based on fiber optic and coding technologies that are capable of immense capacity. On the other hand, the “last mile” local drop to the end user is typically still the legacy copper line installed decades ago for telephone service. Because the legacy copper lines were designed for performance that did not contemplate today's fiber optic capabilities, present copper line technologies cannot avail end users of the high bit rates that modern long haul infrastructure can provide. The user is limited by his local drop connection to the service provider.
Looking at the communications system architectures currently being pursued by service providers, nearly all suffer from implicit assumptions that preserve the notion of connection-based service.
The use of telecommunication resources has moved well beyond mere telephone calls. These voice communications messages are no longer the dominant kind of information flowing through the world's communication networks. Telecommunication users today utilize these resources for many other forms of information. Computer data and video are just examples of the future. Users are requiring that their communication link to the global networks rise to the occasion in terms of bandwidth, that is, digital data rate capability. The legacy links as well as the architecture of the central office (telephone exchange) and its cable to the user cannot deliver the information capability desired for all this data, video and other information. However, a somewhat recent development is the application of digital technology to legacy copper systems—a development variously designated as “xDSL.”
xDSL is a generic term for digital subscriber line equipment and services, including packet-based architectures, such as ADSL, HDSL, SDSL, VDSL, and RADSL. That is, x is the generic. xDSL technologies provide extremely high bandwidth over embedded twisted pair, copper cable plant. xDSL technologies offer great potential for bandwidth-intensive applications, such as Internet access, remote LAN access, video conferencing, and video-on-demand.
ADSL or asymmetric digital subscriber line services generally use existing unshielded twisted pair (UTP) copper wires from the telephone company's central office to the subscriber's premise, utilize electronic equipment in the form of ADSL modems at both the central office and the subscriber's premise, send high-speed digital signals up and down those copper wires, and send more information one way than the other. The ADSL flavor of xDSL services is capable of providing a downstream bandwidth of about 1.5 Mbps-6.144 Mbps, and an upstream bandwidth of about 32 Kbps-640 Kbps with loop distances ranging from about 3.7 km-5.5 km. HDSL or high bit rate digital subscriber line services provide a symmetric, high-performance connection over a shorter loop, and typically require two or three copper twisted pairs. HDSL is capable of providing both upstream and downstream bandwidth of about 1.5 Mbps, over loop distances of up to about 3.7 km. SDSL or single line digital subscriber line services provide a symmetric connection that matches HDSL performance using a single twisted pair, but operating over a shorter loop of up to about 3.0 km. VDSL or very high bit rate digital subscriber line services are typically implemented in asymmetric form, as a very high speed variation on the ADSL theme over a very short loop. Specifically, target downstream performance is typically about 52 Mbps over UTP local loops of 300 m, 26 Mbps at 1,000 m, and 13 Mbps at 1,500 m. Upstream data rates in asymmetric implementations tend to range from about 1.6 Mbps to about 2.3 Mbps. Additionally, there is RADSL or rate adaptive digital subscriber line services. RADSL provides a dynamic connection that adapts to the length and quality of the line.
In the xDSL family of services, many xDSL themes, including ADSL, HDSL, SDSL, VDSL, and RADSL, utilize a packet-based approach that does away with the line-grabbing practice of circuit switched networks, such as ISDN (although ISDN service is a form of digital subscriber line). This packet-based approach is very advantageous in a variety of situations, such as high-speed data services, including high definition television or HDTV transmissions.
xDSL services, also commonly referred to as simply DSL or digital subscriber line services, are much more dependent on line conditions than traditional telephone services. Traditional telephone services typically use a bandwidth including frequencies up to about 3 kilohertz, while the DSL services utilize a bandwidth including frequencies up into the hundreds of kilohertz. While some local loops are in great condition for implementing DSL services, that is, the local loops have short to moderate lengths with minimal bridged taps and splices, many local loops are not as clean. For example, local loop length vary widely, for example, from as short as a few hundred meters to as long as several kilometers